1. Field of the Invention
This invention is an apparatus and process for recovering silver in the form of a silver compound precipitate from certain silver compound-containing solutions, and more particularly to a process for recovering silver in the form of silver sulfide from aqueous photographic solutions such as spent thiosulfate photographic fix and bleach-fix solutions containing soluble silver complexes with thiosulfate, and wash water solutions resulting from washing fixed photographic film during processing (containing as little as 0.01 gram of Ag.sup.+ per 1000 ml). The process is also applicable to recovering silver from other types of solutions such as wash waters from silver nitrate manufacture and photographic emulsion manufacture (containing silver halide), but a thiosulfate compound or other soluble sulfur compound should be added to provide sulfur ions.
2. The Prior Art
U.S. Pat. No. 2,221,163 describes the recovery of silver from waste photographic fix treatment with a guanidine salt or quaternary ammonium compound, along with a base such as sodium or potassium hydroxide to raise the pH to a high value, thus causing silver sulfide to precipitate. The reaction is said to be helped by briefly irradiating the treated fix with ultraviolet light. Waste photographic fix and bleach-fix solutions normally give a pH ranging from 4 to 9, and rapid and complete precipitation with a guanidine salt or quaternary ammonium compound in accordance with the patent allegedly will only occur if enough alkali is added to increase the pH to a value of 10 to 11. A process which does not require chemical additions would be simpler and less expensive, and would be less apt to have a detrimental effect on the ecology when the stripped solution is finally discharged.